Game of Thrones
One of the main protagonist of 8 seasons suddenly went insane and turned the whole city into ashes. The Iron Throne is the aftermath of that which made a lot of fans mad. One of the most popular character with a redemption arc spanning most of the series died in the most puzzling way. The ending didn't make sense.
Breaking Bad
Fly isn't bad. It's a bottle episode which put a lot of fans who are mainly interested in story progression off. But I would argue it's one of the more essential episodes when it comes to developing Walter White's character.
Simpsons
One of Simpsons main downfall is the overuse of celebrities. The Simpsons was no stranger to celebrity cameos, but they really stretched too far in this episode with Lady Gaga. Lady Gaga didn't bring much to the episode and the musical numbers were cringy.
Black Mirror
A shift of tone compared to the rest of the series. It felt like a Disney movie. I think some people might have been put off with Miley Cyrus. I thought she was fine, but I may be in the minority. Previous episodes were more... serious and consequential? This one just felt off compared to other episodes.
Friends
Glorified clip show episode.
X-Files
It felt like B-horror movie without the charm. Predictable plot about some Native American burial curse that have been explored in other shows and movies with much better tact even at the time. X-Files occasionally have episodes like this where it just drop the ball with the story and direction. Can't all be winners.
South Park
Million Little Fibers almost completely ignores the South Park universe and focuses on an annoying minor character who is a towel. It's a parody of Million Little Pieces which has its own host of controversies. Funnybot just had no good jokes.
Dexter
Oh boy where do I begin. After season 4, there was a change of direction and showrunning. In the final season, they completely did not give a fuck. It is one of my favorite hate watches of all time and I would go as far to say it's the worst season finale ever. It's hard to explain why last episode is the worst without explaining last season, but basically: most popular character dies for no reason, main Character Dexter removes himself from society and becomes lumberjack, seven different plot threads completely utterly ignored with no closure, and the Miami PD are still staffed by simpletons who missed a plethora of giant breadcrumbs.
Stranger Things
Went into a weird tangent. Like it's own self contained episode with no progression of the main plot. Introducing several new characters that you'll likely never see ever again. Also doesn't feel like the same universe as the rest of the show. Stranger Things Universe more familiar. Innocent small American town. But the city where this particular episode took place, feels like some Robocop or Judge Dredd esqu dystopia.
Buffy
Propaganda episode telling you that excessive drinking (and drugs???) are bad.
Seinfeld
Clip show episode
Supernatural
Only know a friend who watches the show religiously. But said that particular episode was the worst because it tried to do a spin off.
Law and Order SVU
The infamous episode dealing with the gamergate controversy. I believe there is good potential to make an episode based on the controversy. But instead, what we got a lot of "how you do Kids" moments where they completely missed the mark on what should have been an extremely easy target. In the past, SVU did a good job mirroring current true crime events with good accuracy (well except the episode based on the lacrosse team) nailing down the tone and atmosphere, but this isn't it.
House of Cards
We all know Kevin Spacey left on the last season due to landing in hot water with plethora of sexual assault allegations. They went to the extreme and ignored Frank Underwood character completely after the first episode. Claire Underwood tried to take his place, but unfortunately you can't change horses in midstream after 6 seasons of build up. The last episode was particularly bad because it provided zero closure for many plot threads and the final scene where Claire kills Doug came off as pretentious.
Always Sunny
Filled with flashbacks that made the episode more of a snooze.
Family Guy
Tries to do something new having an episode about characters doing commentary on another episode. Gimmick fell flat. The cutaway gags are probably the worst of any episode period.